The present invention relates to a thermal conductivity measuring device suitably used in an apparatus such as a gas chromatograph for analyzing the components of a gas to be measured.
As a thermal conductivity measuring device in a gas chromatograph, a filament or a thermister is generally used. As an example of the thermal conductivity measuring device using a thin film formation technique or a micro-machining technique, the device proposed by John H. Jerman et al. (U.S. Pat. No. 4,471,647) is known.
A thermal conductivity measuring device 1 is constituted by a detector 2 and a flow path 3 in which the detector 2 is arranged, as shown in FIGS. 5A, 5B, and 5C. The detector 2 includes a base 5 having a hole 4 in its center and consisting of, e.g., silicon, and a membrane 6 covering one end of the hole 4. The membrane 6 includes elongated metal film resistors 7 located above the hole 4 and constituting a heating member, and a plurality of holes 8 formed such that a gas to be measured (sample gas) flows on both sides of the membrane 6. The detector 2 is placed upside-down above a groove formed in the upper surface of a silicon wafer 9 with the membrane 6 being located at a lower position. A small gap between the membrane 6 and the groove formed in the upper surface of the silicon wafer 9 constitutes the flow path 3. Through holes 15.sub.1 and 15.sub.2 connecting the lower surface of the silicon wafer 9 to the flow path 3 are formed on both ends of the flow path 3. In addition, the through holes 15.sub.1 and 15.sub.2 are connected to a flow path (not shown) similar to a flow path 17 formed by a glass plate 10 and a groove 16 formed in the lower surface of the silicon wafer 9 to be vertical to the surface of the drawing. The sample gas guided into the flow path 3 through the through hole (e.g., 15.sub.1) on the upstream side flows along the flow path 3 from right to left and is discharged through the through hole 15.sub.2 on the downstream side. Note that reference numeral 11 denotes a ring disposed on the silicon wafer 9 to house the base 5; 12, a lid placed on the ring 11; and 13, a gasket disposed between the lid 12 and the base 5.
The thermal conductivity of the sample gas is measured as follows. First, a constant current is supplied to a heating element 7 embedded in the membrane 6 or disposed thereon to generate heat. Then, the quantity of heat dissipated from the heating element 7 changes depending on the thermal conductivity of a gas with which the heating member 7 is brought into contact. This change appears as a change in resistance of the heating element 7, i.e., a change in voltage across the heating element 7. Therefore, the thermal conductivity of the gas can be measured from the value of this change.
In the conventional thermal conductivity measuring device having the above-described structure, the detector itself is used as a constituent element of the flow path 3, and the flow path 3 for guiding a sample gas to the detector 2 is integrally formed with the detector 2. With this structure, in comparison with the volume of a hollow portion 14 formed by the hole 4 of the detector 2 and the membrane 6, the volume of the flow path 3 opposing the hollow portion 14 through the heating element 7 is very small, thus posing the following problems.
When, for example, a gas B which is different from a gas A which has been flowing in the flow path 3 flows to the detector 2 through the flow path 3, the gas A remaining in the hollow portion 14 and the gas B in the flow path 3 near the hollow portion 14 are diffused to each other through the plurality of holes 8 of the membrane 6. Since the volume of the flow path 3 is much smaller than that of the hollow portion 14, and the flow speed is set to be very low to prevent the heating element 7 from being influenced by forced convection, substitution of a gas in the flow path 3 near the hollow portion 14 is very slow to occur. For this reason, the concentration of the gas A in the flow path 3 near the hollow portion 14 is quickly increased, and the concentration gradient of the gas A in the hollow portion 14 and in the flow path 3 is reduced to almost zero. Consequently, diffusion of the gas A from the hollow portion 14 into the flow path 3 near the hollow portion 14 is quickly saturated, resulting in slow progress. In contrast to this, the gas B in the flow path 3 near the hollow portion 14 diffuses into the hollow portion 14. However, since the feed rate of the gas B from the flow path on the upstream side into the flow path 3 near the hollow portion 14 is very low, the concentration gradient of the gas B in the hollow portion 14 and in the flow path 3 near the hollow portion 14 is quickly reduced, and diffusion of the gas B from the flow path 3 near the hollow portion 14 into the hollow portion 14 is quickly saturated, resulting in slow progress. Therefore, the gas A in the hollow portion 14 cannot be smoothly substituted with the gas B, and the gas A also remains in the flow path 3 near the hollow portion 14 in a large quantity, resulting in a large error in measurement of the gas B owing to the residual gas A.
In the use of the device for a gas chromatograph, when a sample gas carried by a carrier gas flows to the detector 2, the carrier gas remaining in the hollow portion 14 of the detector 2 is not smoothly substituted with the sample gas at a position near the heating element 7. In addition, if the flow path 3 is narrow, the amount of sample gas which can flow in the flow path 3 is small. Consequently, the flow of the sample gas is stopped before the carrier gas remaining in the hollow portion 14 of the detector 2 is substituted with the sample gas, and the sample gas to be measured is further diluted with the carrier gas, causing a large measurement error.
Note that the substitution efficiency may be slightly improved by increasing the flow speed in the narrow flow path 3 or increasing the flow rate. However, the quantity of heat dissipated from the heating element 7 is increased by the flow of a gas (force convection), resulting in a large error in thermal conductivity measurement.